This invention relates to photometric monitoring devices in general, and more particularly to detection and surveillance devices for objects, for example, intruders, entering or leaving a monitored area.
Previous photometric monitoring devices may be divided into two classes, those in which a constant light source is spaced from a photocell so that an object passing therebetween breaks a light beam, and those in which a monitored light source is converted to an analog signal. The constant light beam devices have serious limitations imposed thereon inasmuch as they must be fairly permanent installations and may monitor extremely small areas, such as window and door openings of building. Analog photometers suffer the disadvantage of being unable to respond adequately to varying ambient light conditions and light changes which take place some distance from the device. Furthermore, since they must be set to trigger at a predetermined level, such devices are unreliable and may trigger on unwanted light changes while failing to respond to desired light changes under certain conditions.